The invention relates to a switching arrangement for telecommunication systems, more particularly telephone switching systems.
There are known switching arrangements wherein in through-switching components ready for seizure the idle condition is marked by an idle potential applied to a check point over resistors of a seizing circuit of the switching component in question. The busy condition is marked by the absence of that potential. The busy/idle condition (free or blocked) of a switching component may be ascertained by using a switching device having an internal impedance corresponding to that of a preceding switching component (e.g., a selector) testing for seizure over a test and seizing circuit completed by connecting the test switching means to the check point. Blocking of a seized switching component due to seizure by the preceding switching component is brought about by applying an opposed potential differing from the idle potential over auxiliary test switching means in the test and seizing circuits provided in the preceding switching component.
A switching arrangement of the aforementioned type has been disclosed in the prior art by West German Pat. No. 1,173,538. It is a commonly known technique for switched telephone systems to ascertain the busy/idle condition, i.e., free or blocked seizure wires of selectors, repeaters and similar equipments using test relays which initially test to comparatively high impedance. If the equipment being tested is in the idle condition, it is caused to be seized and blocked to a low impedance. In the idle condition the equipment being tested offers a potential (e.g., negative potential) identifying the condition over its seizing circuit at a check point allocated thereto.
A testing selector works from its test circuit with an inverse potential, e.g., a positive potential (ground potential). If a selector finds an idle output, that is, if a test circuit of a testing selector is connected to a seizing circuit of a following free equipment whose check point carries the potential identifying the idle condition, the test relay of the selector responds, stops it and connects the inverse potential with a relatively low impedance to the check point of the seizing circuit of the equipment so seized. Therefore, the potential identifying the idle condition is eliminated. The test relay of the selector is held over the circuit so completed. Other selectors testing thereafter almost meet with the inverse potential, to be precise, a partial potential across the seizing circuit and thus recognize the equipment concerned as seized.
Thus, there is a divided circuit when a seized equipment is being tested. The common part of the divided circuit consists of the seizing circuit of the seized equipment. One of the two branches of the divided circuit comprises a blocking circuit of the selector that has previously seized the equipment being tested. In this branch is located a low-resistance coil of the test relay of the selector. The other of the two branches is the test circuit of the other selector which finds the equipment being tested blocked (seized). The low-impedance and the high-impedance coils of the test relay of the second selector are connected in series in this branch. The junction point is the check point of the seizing circuit of the equipment being tested. Because the blocking circuit of the first selector is substantially more low-impedance than the test circuit of the second selector, one gets a branching-off of the current at the common check point. The splitting of the total current in the seizing circuit of the equipment being tested into two partial currents is so devised that the predominant part of the total current energizes the blocking circuit and the test relay of the first selector. The partial current in the test circuit of the second selector and in its two test relay coils is so weak that the test relay cannot respond. Thus it is ensured that the test relay of the first selector continues to respond and the test relay of the second selector cannot respond.
More difficult, however, is the special case of the parallel testing of two selectors. In this case, both test circuits of two independently testing selectors are connected randomly and simultaneously to the check point of a subsequent equipment. Assuming that the test relay of both selectors have identical resistance values, which is the rule in many cases, a current division occurs in the ratio 1 to 1. That means that half the current in the seizing circuit of the subsequent equipment energizes the coils of the test relays of both selectors. In this case both test relays must not respond. This condition is difficult to comply with. Therefore, the low-impedance blocking by the test relay that first operates its contacts shall prevent the other test relay from responding also.
To improve the conditions during the parallel testing of two selectors with a view to preventing double testing it is common knowledge to provide prior switching arrangements (e.g., West German Pat. Nos. 1,013,701 and 1,165,678) with two test relays per selector, of which a first rapidly responding test relay used to stop the selector turns on a second test relay (auxiliary test relay). The second test relay serves to ensure the prevention of double testing of two selectors in the parallel test.
West German Auslegeschrift No. 1,940,847 further discloses a technique for connecting seizing circuits to high impedance after their seizure. The same publication also discloses a technique for connecting inverse potentials not only from a testing and seizing selector but also within an equipment seized by a preceding selector over its own resistor to its own seizing circuit. This principle is realized, among other things, through the use of an opposing winding of the seizing relay. The opposing winding is substituted for the resistor mentioned earlier. This opposing winding is designed such that the seizing relay receives adequate holding current. Upon releasing the preceding selector, the current in the opposing winding is decreased so that the seizing relay is locked out by opposing energization. By switching the resistor or the opposing coil into circuit, the current in the test circuit of the blocking circuit is decreased, but not in the seizing circuit. The voltage at the check point of the equipment concerned is moved towards the reverse voltage. Hence, for high-speed selectors having highly sensitive test relays there usually exists for the insertion of the resistor or opposing coil a time condition that ensures that the test relays of the selectors can first respond safely before the current in the test circuit is decreased. To meet this time condition it is a common technique to employ auxiliary relays with response lags.
In all these known instances there is the danger of double testing within danger periods caused by the test and seizing procedures. The principle of using two test relays (test relay and auxiliary test relay) cannot be introduced belatedly in existing older automatic exchanges. Moreover, the danger of double testing is increased from the viewpoint of resistance tolerances and contact junction resistors in the test and seizing circuits.
Accordingly, an object of the invention is to provide means for ensuring the protection against double testing during the parallel testing of two selectors, whereby particularly, the parallel operation of selectors having test circuits of different structures must be taken into consideration. Likewise, danger periods of a double testing must be avoided.